test that nonexistant text will throw error

app/Plan/UseCases/CreatePlan.php

@@ -49,6 +49,9 @@ class CreatePlan
         }
         $textId = $request->textId;
         $text = $this->textRepo->find($textId);
+        if ($text === null) {
+            throw new DomainException("Text with id: $textId doesnt exist");
+        }
         $nodesOfText = $this->filterForNonParentNodes(
             $this->nodeRepo->findByTextId($textId)
         );